Data storage on rotating disks dates back to the development of the first disk phonograph. Elemental to disk data storage is a mechanism for rotating the disk at a desired angular velocity.
Spindle motors have been included within disk spindle hub and bearing assemblies. This common arrangement has the advantage that the motor and the disk spindle share a common spindle shaft and bearing assembly. Also, there is no likelihood of belt slippage since there is no drive belt between the spindle and the motor.
Conventional spindle motors for hard disk drives have typically included a rotating annular permanent magnet structure in surrounding proximity to a fixed, multiple pole stator structure comprising pole pieces facing the rotating permanent magnet. The stator pole pieces have been formed of laminations of soft magnetic core material. Coils of wire have been formed around narrow gap spoke segments of the stator core. Typically, six or nine coils and poles have been provided in three-phase brushless DC spindle motors, and they have conventionally confronted 4, 6, 8, 9, or 12 pole permanent magnet rotors. The nine-pole stator structure --eight pole rotor magnet arrangement has been particularly popular in fixed disk drives because the uneven number of stator and rotor poles has been said to reduce generation of uneven torque force in the spindle, commonly referred to as "cogging torque". One example of this conventional arrangement of stator and rotor poles in a hard disk drive spindle motor is given in U.S. Pat. No. 4,858,044. One drawback of these conventional disk drive spindle motors is that they are complicated and expensive to assemble during disk drive manufacturing. Another disadvantage of "low cogging torque" disk spindle motors of the type employing 8 pole-9 slot arrangements is that uneven torque force results, leading to vibration and noise.
Tin-can or "canstack" motors have commonly been employed as stepper motors. These low cost motors are typically formed of two stacked annular stator coil bobbins surrounded by a "can" of soft magnetic material which forms a series of circumferentially alternating magnetic pole tabs along an inner opening. The polarity of a particular tab is governed by direction of current flow through its associated bobbin coil. A rotor includes a shaft and a permanent magnet structure. The permanent magnet structure defines a series of circumferentially spaced apart poles. A skew exists between the pole tabs of the stators and the permanent magnet poles, such that a North pole of a pole tab will directly align with a South pole of the permanent magnet at one detent position. However, at this position, the other tabs and poles are misaligned. By progressively alternating the direction of current flow in the two bobbin coils, unidirectional rotation in steps of the rotor shaft is realized.
As explained, the rotor shaft is journalled to the "can" and is rotated as direct current driving pulses are passed through the bobbin coils. Driving currents applied to the bobbin coils cause a step-by-step rotation of the rotor along stable detents, and thus render these motors suitable for incremental positioning, such as rotation of a printer platen or print head, or head actuator positioning in a floppy disk drive. Examples of conventional canstack DC motors including bobbin wound coils are given in U.S. Pat. No. Re 28,075, and in U.S. Pat. No. 3,238,399, for example. Bobbin coil permanent magnet motors have also been employed as synchronous motors within small cooling fans used to draw cooling air currents through or across heat-generating components of electronic and computing equipment, such as switching power supplies, for example.
While these technologies have been proven to be useful in several applications, a hitherto unsolved need has remained for a significantly cost-reduced motor for a disk spindle of a low cost hard disk drive.